"Clearing the Skies of Texas," Feature Article, September 2004

clearing the skies of Texas

A new technology promises to reduce the emissions from gas-fired power plants.

This article was prepared by staff writers in collaboration with outside contributors.

Texas is known for wide-open ranges and long horizons. But in April, the U.S. Environmental Protection Agency cited Dallas, Houston, and San Antonio among the American cities that exceed national standards for ozone pollution. Ozone, a major source of smog, forms as nitrogen oxides react with sunlight.

To try to ameliorate the situation, the Texas Commission on Environmental Quality has enacted tough regulations for power plants in the state. Power plants in counties that exceed air pollution standards must slash emission of nitrogen oxides by 88 percent by May of next year.

That's a tall order, and for TXU, the Dallas-based utility with 17 gas- and four lignite-fired power plants in Texas, that meant finding a way to cut emissions from plants in the Dallas-Fort Worth area—and to do it in a cost-efficient way. To see if there was a way to cut NO_x emissions from its gas-fired plants, TXU teamed up with John Zink Co.'s Todd Combustion Group of Shelton, Conn., to test a combustion technology that had yet to be tried in a utility-scale generation facility.

The solution that the Todd engineers wanted to test was a flue gas recirculation system that differs significantly from conventional systems, which bring in the flue gas with the intake air. The new system, called COOLfuel, instead mixes the flue gas with the fuel stream, to create a gas that has a much lower energy density.

New technology has cut NO_x emissions from this gas-fired power plant by 70 percent.

The advantage is that this leaner fuel mix should produce significantly lower NO_x emissions. "It's a direct function of temperature," said Tim Webster, an engineer at John Zink in Tulsa, Okla., who helped oversee the installation and test run of the system. "The lower the temperature, the less NO_x you produce."

And because the system is designed to enable the force of the fuel stream to pull along the flue gas, recirculation can be accomplished without the use of fans, simplifying operations and minimizing operating costs.

The technology dates back to the mid-1990s, Webster said. Todd Combustion had tested it on smaller boilers, but the system had never been applied on such a large scale.

The Collin Station power plant is a nearly 50-year-old 160-megawatt unit that provides power during heavy loads. The COOLfuel mixing devices were placed just upstream from each of the 40 burners in the plant's furnace, with the hope that, by installing the mixers as close to the burners as possible, the maximum amount of flue gas recirculation could be achieved.

The first results were very encouraging: Even before the equipment had been optimized, NO_x emissions had been cut in half. It was later determined that air was leaking through some seals, limiting the NO_x reduction, and resulting in lost production capacity.

Through computational fluid dynamics modeling, modifications were made to the gas nozzles and flue gas connections during normal shutdown periods to try to reduce the pressure drop due to the introduction of the flue gas. Even more modifications were made based on airflow observations of a one-sixth-scale model of the ductwork. On the basis of this modeling work, baffles were added to the interior of the air ducts to balance the distribution of the combustion air to each burner.

Finally, the gas jet nozzles were redesigned, optimizing the port design and customizing each nozzle based on its location in the boiler.

Once these changes were complete, the NO_x levels dropped even more and the lost production capacity was recovered. Although some configurations enabled the NO_x levels to go as low as 20 parts per million, the plant was able to run at a full load consistently while emitting 41 ppm, a 70 percent reduction from the pre-installation levels with no appreciable impact on the plant's operations.

While it is expected that coal-fired power plants will be the focus of the push to lower NO_x emissions in Texas, utilities are likely to retrofit their gas-fired plants too.

Webster estimates that the Todd Combustion system can be installed for just $10 to $15 per kilowatt. That's a fraction of the cost of a conventional selective catalytic reduction system found in many plants.

It's enough to make the skies of Texas a little brighter.